Arsenic's toxicity to man and other living organisms has led to serious environmental problems and difficulties in procuring suitable drinking water in many parts of the world. In well-oxidized waters, arsenic is present predominately as arsenate (H2AsO4−1 and HAsO4−2) while under reducing conditions it is usually present as arsenite (H3AsO3 and H2AsO3−1) [1]. Since the reduction and oxidation reactions of arsenic are particularly slow, both of these oxidation states can coexist irrespective of the redox conditions [2]. The arsenic (III) species are 25-60 times more toxic than arsenate and are more mobile in the environment [3]. Absorption on mineral surfaces is an important factor that controls the mobility and bioavailability of arsenic. Arsenate adsorption on clays and aluminum and iron oxides is greatest at low pH and decreases with increasing pH while arsenite has a maximum in adsorption to these materials at approximately pH 8.5 [4].
A large variety of materials have been tested for removal of arsenic from water including adsorbents such as phyllosilicates, silica, and hydrous oxides of iron and alumina [3]. The most successful and heavily investigated materials have been iron oxides, especially ferrihydrite [5-12].